This invention relates to nuclear magnetic resonance (NMR) apparatus. More specifically, this invention relates to radio frequency (RF) coils useful with such apparatus for transmitting and/or receiving RF signals.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with the tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated the use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.0, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.0, the aligned magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the Larmor equation .omega.=.gamma.B in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.0 plus other fields) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.0, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1 are applied orthogonal to the direction of the B.sub.0 field by means of RF pulses through a coil connected to an RF transmitting apparatus. Under the influence of RF excitation, magnetization M rotates about the direction of the B.sub.1 field. In NMR studies, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.0 field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane precess around the direction of the static field. The vector sum of the spins forms a precessing bulk magnetization which can be sensed by an RF coil. The signals sensed by the RF coil, termed NMR signals, are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. In NMR imaging applications, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the signals. This information is later used to reconstruct images of the object studied in a manner well-known to those skilled in the art.
In design of RF coils for use in whole-body NMR studies, it is advantageous to limit radiative RF magnetic field loss by containing the field within the imaging volume, to the extent possible. Additionally, it is known that RF coils employ capacitance appropriate for the given RF coil inductance to attain the desired resonant frequency. In this regard, it is desirable to permit adjustment of the capacitance while expending minimum time and effort in the task. U.S. Pat. No. 4,751,464 (the "'464 patent") to Bridges and U.S. Pat. No. 4,746,866 (the "'866 patent") to Roschmann recognize these design goals, but are limited in meeting them.
Referring to the '464 patent, a cavity resonator comprises a plurality of transmission lines aligned parallel to a common axis and magnetically coupled to produce an essentially uniform dipole magnetic field perpendicular to the axis. The transmission lines comprise two conductor lines, the first conductors being arranged inwardly of a second conductor common to all lines. The common conductor is concentrically arranged about the axis of the resonator, and constitutes a cylindrically-shaped shield to help contain the magnetic field within the cavity. However, the shield is open at its two ends, permitting undesirable radiative loss of magnetic field through the open ends.
The transmission lines of the '464 patent include capacitance for tuning to resonance. However, distributed capacitance is used, which is best understood by knowing what it is not--lumped element capacitance. Lumped element capacitance is provided by discrete capacitors. Distributed capacitance does not use discrete capacitors; rather its capacitance is inherent to design structure, meaning the capacitance is distributed across various parts of a structure which together provide electrical capacitance. In the '464 patent, those "parts" providing distributed capacitance consist of the ends of the first and second conductors which are bent to form a pair of plates having a dielectric disposed therebetween.
Impedance matching to resonance is well known to those skilled in the art, and yet to achieve the desired resonant frequency, one requires a precise amount of capacitance. As a matter of manufacturing practicality, this is a difficult requirement to meet when using distributed capacitance. To understand why, it is necessary to consider the nature of distributed and lumped capacitances. Lumped capacitance is provided by one or more discrete capacitors having a precise quantity of capacitance (within a selectable tolerance range). The nature or purpose of discrete capacitors consists, solely, of providing capacitance.
In contrast, the nature of distributed capacitance is multifunctional, as the structure providing distributed capacitance also serves as mechanical support. For example in the '464 patent, shield 12 and conductors 18 serve: 1) a mechanical support function, with its own design requirements; and 2) a distributed capacitance function, also having specific design limitations. It is difficult to manufacture structures serving such dual purposes, one of which is the provision of a precise quantity of capacitance. This is one of the inherent problems to the distributed capacitance offering of the '464 patent.
Another problem is that the distributed capacitance of the '464 patent is not adjustable. Variable capacitance is useful in a number of circumstances. For example, if the quantity of capacitance provided is incorrect for resonant operation at a desired frequency, then adjustment is desired. Similarly, if over time the resonant frequency changed due to structural changes in the resonator, then one would want to adjust the capacitance. Nevertheless, the distributed capacitance provided by the '464 patent is fixed, and often incorrectly so due to manufacturing complexities mentioned above.
The '866 patent attempts to solve the latter problem by providing variable distributed capacitance. Specifically, the '866 patent includes an outer conductor in which there is arranged a dielectric. Two inner conductors are arranged, in the dielectric. One or more of the inner conductors are displaceable, thereby forming a variable distributed capacitance between the inner displaceable conductor(s) and the outer conductor. The system of conductors is tuned or readjusted by displacement of one or more of the inner conductors.
While the '866 patent offers variable capacitance, it too has shortcomings. Specifically, in order to establish a desired capacitance for resonant operation, one needs to know where to move the inner conductor to create such capacitance. Additionally, even if the inner conductor is moved into the desired position, it needs to be held in place to maintain the proper capacitance for resonant operation. As a practical matter, this may be a difficult achievement because the resonator vibrates during operation, causing undesirable relative motion between the inner and outer conductors, thereby changing the distributed capacitance from that which is required for resonant operation.
What is needed is an apparatus which reduces radiative losses of the RF magnetic field, and which provides lumped element capacitance for resonant operation.